U.S. Attorney General Eric Holder is right that the criminal sentencing system is broken in this country. Holder has only limited powers to fix it, however. The rest is up to Congress, which is largely to blame for the problem we now face.

In a speech to the American Bar Association on Monday, Holder used his bully pulpit as the nation’s top law officer to focus attention on the staggering U.S. prison figures and to call for changes that would stem the tide of incarceration.

The most staggering example: Since 1980, at the same time this nation’s population has grown by one-third, the federal prison population has grown by 800 percent.

As a result, Holder noted that the United States, with just 5 percent of the world’s population has nearly one-fourth of the world’s prisoners.

Much of that increase is directly attributable to drug prosecutions.

About half of the 219,000 inmates in federal prisons were convicted of drug-related crimes. Many carry mandatory-minimum sentences set by Congress, which took discretion away from judges who have no choice but to send low-level offenders to prison for decades.

This has had an especially profound and disproportionate effect on young, black men in this country.

The attorney general can’t do much about sentencing statutes, but he can control prosecutions, and Holder announced that he has established new guidelines to rein in U.S. attorneys under his supervision. This is long overdue, because federal prosecutors hold enormous power over suspects who are pressured to accept plea deals to avoid going to trial and risking an even longer sentence.

Holder’s narrowly crafted guidelines still allow prosecution of low-level offenders, but nonetheless, this is a welcome message from the top that it is time for U.S. attorneys to throttle back.

The problem is that Holder will be around only as long as his boss is in office, and future attorneys general could go back to business as usual.

Only Congress has the ability to reduce the number of people sent to prison over the long term. That will require a change in thinking among lawmakers who have steadily expanded the number of federal crimes and the severity of sentences in the dubious war on drugs.

Holder’s frank acknowledgment that this nation sends too many people to prison also means that tens of thousands of people do not belong there now.

Something should be done about that. It can be done: California, once the nation’s leading proponent of locking up people, is now under order to reduce its prison population.

Congress and other states should follow this example. It is not enough to just curtail unnecessary prosecutions and mandatory sentences unless something is done about past injustices, too.